The present invention relates to spring motors and, more particularly, to a spring motor for delivering constant torque for driving an electric power generator.
There has been a recent interest in development of power systems for electronic devices such as flashlights and radios which can obtain electrical power from mechanical sources. These systems are of particular interest for use in emergency kits in which storage batteries may be depleted or in remote locations where access to electric generators is limited.
U.K. Patent Specification No. 9516144.4 describes a radio which is powered from an electric generator driven by a spring motor. The spring motor comprises a strained steel band which is wound on a torque drum and is then allowed to unwind onto a storage drum. The torque drum is mounted on a shaft which rotates with the drum and is connected to drive an electric power generator. The spring force of the wound spring is released at a constant angular rate so as to drive the generator at a constant speed and produce a constant voltage output. However, although the spring force is constant, the torque developed by the spring motor, which is a product of the spring force and the radius of the spring on the torque drum, varies as the spring unwinds. Since the power developed by the spring motor is proportional to the product of torque and angular frequency, the power output varies as the spring unwinds from the torque drum. When such a variable power source is used to drive a generator, the electric power output of the generator will likewise vary and affect operation of devices receiving the electric power.
Notwithstanding the above described power variation, it is desirable to assure that the power available from the spring motor, even in the last few turns, is sufficient to provide the minimum amount of power necessary to operate electrical devices connected to the electric power generator. Typically, this would be achieved by oversizing the spring motor to produce the desired torque at t he last few turns. Such design, however, will produce excess torque at all other spring conditions and lead to wasted power and extra cost. Accordingly, it is desirable to provide a spring motor which can produce constant torque at any winding state of the spring.